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Deworming--positive FEC after PowerPak

Not my horse. Did a FEC for a friend, count was 1900. Horse had been at a boarding barn on rotational worming protocol until last fall. In the fall went home with owner. Since then owner did an ivermectin and one other unknown dewormer. Fecal in April was 1900. Owner completed PP 12 days ago. Did FEC today. Result 137.

What does this mean? Shouldn't it be negative? Is horse resistant to fenben?

Isn't a powerpak supposed to clear everything out? I've seen horses with a zero count on a lot less than that.

Fenbendazole is not 100% effective on every parasite at all life stages. So no, it won't "clear everything out". Some horses are "better" hosts than others, and FECs will vary from one day to the next in the same horse, which is independent of deworming.

I would ask JB but I think you should wait and do another FEC in 14 weeks. I know after using QuestPlus you should wait 16 weeks.

I would not worry about that low number. Some horses are high shedders and need deworming more that others. Right now it's too soon to tell if your friends horse is a high shedder or not. I also came from a barn that was on a rotational program that did not work for my horse. I can now manage him myself and have learned that he's a low shedder and manages the parasites well on his own.

Thanks, crownedragon! Isn't that the same as having developed a level of resistance?

Is the next step to treat with a different class of dewormer? And can he do that right away?

Why not consult with her vet for what is best for your area?

I did go thru something similar with one of my horses. I kept having high (sometimes crazy high, sometimes just high) counts. I did a power pac and when we did the next FEC she was a ton better but certainly not zero.

Some horses are higher shedders than others.

My mare it has been two years of very regular FECs and deworming and the last two FECs have been very low or zero. No idea what has changed but I will take it.

Here's a good article on The Horse (sponsored by Pfizer) about FECs and strategic deworming that touches on this point. In general, it is thought that a low parasite burden is helpful for building/maintaining immunity against parasites, and perhaps in moderating the immune response against other pathogens. Perhaps letting horses deal with their own parasites somewhat, is akin to the "hygeine hypothesis" in humans, wherein our children do not develop proper immune responses if not faced with challenges during their formative years.

"Fortunately, total eradication of fecal egg shedding in horses on pastures is not a necessary goal for a parasite protection plan. This is good news, because it's also nearly an impossible goal to reach. In fact, there is little evidence to show that a continuous zero-EPG standard is needed for the normal development of foals or the well-being of adult horses.

Parasitologists generally agree that the proper objective of parasite control is to maintain the parasite burden at a low level, rather than to eliminate parasites entirely. This middle ground avoids over-treatment, limits the cost of parasite control, and helps horses maintain partial immunity to overwhelming infection. In other words, it's a good idea to allow a very low level of parasite infection so that horses' immune systems can learn to deal with these invaders if they occur in larger numbers."

That other thread is a good one, but to add more context to this... you might feel better about this if you knew a bit more about how the PowerPac operates chemically. The reason it's a five-day double dose is because first you have to get the encysted strongyles to erupt into the gut (because, by definition, they are encysted in the gut), then hit them again with more fenbendazole so that they actually die. ou essentially have to hit any given encysted strongyle twice: once while it's encysted, and once when it's floating around in the gut.

So at least in theory, you could get a small number of encysted strongyles that "snuck through," meaning they erupted into the gut but didn't encounter that double dosage concentration of Powerpac again when they got into the gut.

This is a different mechanism than Quest (moxidectin), which dissolves the encysted strongyles in situ without any eruption into the gut. The advantage, obviously, is that this is pretty darn effective. The disadvantage is that moxidectin can only do this for one of the two larval, encysted stages of strongyles (so you're leaving one stage behind). But the PowerPac isn't a magic bullet either since it takes a double whammy punch to work, and there is evidence of fenbendazole resistance among strongyles in certain parts of the country.

There's also the much simpler explanation, which is that you've got strongyles in your pasture area. Strongyles live part of their lifestyle outside of the horse's body, waiting to get ingested. So if you've got strongyles on the farm, you could theoretically run a fecal on a horse right after finishing a Powerpac and still get a positive from the newly-arrived strongyles in the horse's gut.

Melhorse: part of the puzzle is that the horse has only been on these pastures since Nov and NO horses were kept there before, a new horse owner. His other horses had zero counts before the PP (so they did not get the PP).

I read JB's response on the other thread, and although I referred him to vet, I think I will also send him the link on the thread.

. Fecal in April was 1900. Owner completed PP 12 days ago. Did FEC today. Result 137.

What does this mean?

It means the horse has an acceptable load of strongyles (I'm assuming that's all that makes up the 137) and does not need to be dewormed right now. But,given the 1900, I WOULD get another FEC in 2-4 weeks just to see what's going on.

Shouldn't it be negative?

The goal for horses is never to 100% clean them out. They NEED a light load to constantly keep the immune system up and ready.

Besides, 1900 was for strongyles (I'll assume the horse didn't have ascarids). Fenbendazole has a very high and widespread resistance with adult strongyles. A Power Pack goes primarily after encysted strongyles, which were no part of that FEC. As a sort of side effect, despite it's resistance, at that dosage for that length of time, it's bound to take care of a great many adult strongyles. So yes, it IS expected that it did not kill them all.

Is horse resistant to fenben?

dewormers don't act on horses, they act on the parasites, and yes, see above.

I would ask JB but I think you should wait and do another FEC in 14 weeks. I know after using QuestPlus you should wait 16 weeks.

For this particular horse, given the really high 1900 count, I personally would want another FEC in 2-4 weeks. If this horse just has a weaker parasite immune system, he may not have what it takes to keep numbers low enough. It's concerning he was at 1900 if he'd been on a "regular deworming program" which I am *assuming* (and yes, I know!) included at least a couple of ivermectins in the last 9 months or so. So, either they weren't using ivermectin regularly enough (ie maybe only once a year) and were nearly always using fenbendazole or pyrantel pamoate, or were never giving a full enough dose thereby always underdosing him.

I would not worry about that low number. Some horses are high shedders and need deworming more that others. Right now it's too soon to tell if your friends horse is a high shedder or not. I also came from a barn that was on a rotational program that did not work for my horse. I can now manage him myself and have learned that he's a low shedder and manages the parasites well on his own.

I wanted to add one of the links that JB shared in a thread (Genesee Valley Vet (?)) suggested not doing a fecal within 2 weeks of a deworming because it would skew the results.

This happened to me. One of the horses here was dewormed and a fecal done 12 or 13 days after the deworming. According to this boarder's vet, the horse was "loaded with worms". The information came second hand from the boarder who isn't well versed in horses so I'm not sure how much he got right. He said the fecal showed the presence of "large and small something" (I'm guessing he meant strongyles). The vet he uses is not the most organized, and I'm wondering if the fecals perhaps got mixed up with someone elses.

I was stumped, so I had my vet do a fecal on all of the horses here a week later. All the fecals were negative.